Solid-state NMR study of metastable immiscibility in alkali borosilicate glasses

In several series of lithium, sodium, and potassium borosilicate glasses whose compositions traverse known regions of liquid-liquid phase separation, we have applied triple-quantum magic-angle spinning (3QMAS) B-11 and O-17 NMR to obtain high-resolution information about short-range structure and connections among various network structural units, and their variation with composition and thermal history. Oxygen-17 3QMAS spectra reveal changes in connectivities between silicate and BO3 (B-[3]) and BO4 (B-[4]) units, by quantifying populations of bridging oxygens such as B-O-B, Si-O-B and Si-O-Si, and of non-bridging oxygens. B-[3]-O-Si and B-[4]-O-Si as well as B-[3]-O-B-[3] and B-[4]-O-([3]) B linkages can be distinguished. B-11 MAS and 3QMAS at a magnetic field of 14.1 T allow proportions of several borate units to be determined, including B-[3] in boroxol ring and non-ring sites and B-[4] with 3 versus 4 Si neighbors. By combining the B-11 and O-17 NMR results, detailed information on SUB mixing in sodium borosilicates can be derived, showing, for example, that B-[4] and non-ring B-[3] tend to mix with silicate units, while ring B-[3] is mainly connected to borate groups. In a preliminary study of the effects of varying alkali cation, potassium-containing glasses are similar to those in the sodium borosilicate system, but a lithium borosilicate seems to exhibit considerably greater chemical heterogeneity. In annealing experiments that converted an optically clear to obviously phase-separated glasses, the ratio of B-[3] to B-[4] does not change significantly, but part of the non-ring B-[3] converts to ring B-[3] as the degree of unmixing increases. (C) 2003 Elsevier Science B.V. All rights reserved.